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6.1.1.2. IP DiffServ (DSCP)
DiffServ (DSCP)
Differentiated Services or DiffServ is a computer networking architecture that specifies
a simple, scalable and coarse-grained mechanism for classifying, managing network traffic
and providing Quality of Service ( QoS ) guarantees on modern IP networks. DiffServ can,
for example, be used to provide low-latency, guaranteed service ( GS ) to critical network
traffic such as voice or video while providing simple best-effort traffic guarantees to non-
critical services such as web traffic or file transfers.
Differentiated Services Code Point
(
DSCP
) is a 6-bit field in the header of IP packets for
packet classification purposes. DSCP replaces the outdated IP precedence, a 3-bit field in
the Type of Service byte of the IP header originally used to classify and prioritize types of
traffic.
When using the DiffServ priority mechanism, the packet is classified based on the DSCP
field in the IP header. If the tag is present, the packet is assigned to a programmable egress
queue based on the value of the tagged priority. The tagged priority can be designated to
any of the available queues.
Version IHL
Type of Service
Total Length
Identification Flags Fragment Offset
Time to Live Protocol Header Checksum
Source Address
Destination Address
Options Padding
Example Internet Datagram Header
IP Header Type of Service: 8 bits
The Type of Service provides an indication of the abstract parameters of the quality of
service desired. These parameters are to be used to guide the selection of the actual service
parameters when transmitting a datagram through a particular network. Several networks
offer service precedence, which somehow treats high precedence traffic as more important
than other traffic (generally by accepting only traffic above certain precedence at time of
high load). The major choice is a three way tradeoff between low-delay, high-reliability,
and high-throughput.
Bits 0-2: Precedence.
Bit 3: 0 = Normal Delay, 1 = Low Delay.
Bits 4: 0 = Normal Throughput, 1 = High Throughput.
Bits 5: 0 = Normal Reliability, 1 = High Reliability.
Bit 6-7: Reserved for Future Use.
Bit 0 1 2 3 4 5 6 7
PRECEDENCE D T R 0 0